Monarch Butterfly Migration Rebounds, Easing Some Fears

Monarch butterflies last year in Mexico. They fly more than 2,500 miles south each year from Canada and the United States.CreditCreditRebecca Blackwell/Associated Press

By Victoria Burnett

Feb. 27, 2016

MEXICO CITY — After years of being ravaged by severe weather and shrinking habitats, the monarch butterflies hibernating in the Mexican mountains rebounded last year, kindling cautious hope that one of the insect world’s most captivating migrations may yet survive.

The World Wildlife Fund said at a news conference here on Friday that the orange-and-black butterflies, which fly more than 2,500 miles each year from Canada and the United States to a cluster of mountain forests in Mexico, covered about 10 acres this winter, an area more than three times as large as the space they covered last year.

“We are seeing the beginning of success,” said Daniel Ashe, director of the United States Fish and Wildlife Service, who was in Mexico for the presentation. “Our task now is to continue building on that success.”

The butterflies arrive in Mexico starting in October and spend the winter clinging to branches in the forests of Michoacán State and Mexico State.

During their migration, the butterflies breed along the way, and the journey is completed by their offspring. It is unclear how new generations know how to reach the same spot each year in Mexico. But dwindling amounts of milkweed — on which the butterflies depend for food and to lay their eggs — as well as erratic weather patterns and illegal logging in Mexico have led to a decline in their population and fears that the migration may end.

Alejandro del Mazo, chairman of the National Commission of Natural Protected Areas, said that, based on the span of forest the butterflies covered this winter, the monarch population in Mexico might have risen to about 140 million, a substantial increase from an estimated 35 million at their nadir two years ago.

Omar Vidal, director general of the World Wildlife Fund in Mexico, said the migration was in “the recovery stage,” but cautioned that last year’s numbers were still among the lowest — at their peak 20 years ago, the butterflies covered 45 acres of forest. It was too early to know whether the new figures were a blip.

“Insects are very susceptible to these ups and downs,” he said, referring to swings typical in insect populations.

Scientists and environmentalists said a variety of factors had contributed to the increase: new plantings of milkweed, mild weather and efforts to protect the Mexican forests where the insects rest from illegal logging.

The United States is trying to replace about 7.5 million acres of milkweed either by planting or by stopping the use of pesticides that destroy it. Mr. Ashe said Wednesday that the area of milkweed increased by about 250,000 acres last year.

Mexico has struggled to control logging in its forests. The government’s environmental watchdog, Profepa, has given uniforms and equipment to hundreds of local farmers to keep watch in a 140,000-acre reserve that straddles most of the butterflies’ hibernation spots.

Tierra Curry, a senior scientist at the Center for Biological Diversity, said that saving the butterflies would require more extensive conservation efforts in both the United States and Mexico.

The Center for Biological Diversity has asked the Fish and Wildlife Service to classify the monarchs as “threatened,” said Ms. Curry. Such a classification would lead to a recovery program with federal funding for American states. The population would need to grow substantially to survive storms like the one in 2002 that killed about 500 million butterflies, she said. It also faced a growing problem in Mexico, where warming weather could alter its winter habitat.

“The forest in Mexico is the perfect balance of hot and cold,” she said. If a warming climate hurts that balance, she said, “their winter habitat becomes incredibly vulnerable.”

A version of this article appears in print on , on Page A11 of the New York edition with the headline: Monarch Migration Rebounds, Easing Some Fears. Order Reprints | Today’s Paper | Subscribe